1. Field of the Invention
The invention relates to methods and apparatus for monitoring air quality, in particular for identification of airborne molecules and concentration measurements, for example the emissions of a particular stack of a manufacturing plant. A combination of infrared absorption and emission spectroscopy is effected by an automatically controlled spectroscopic device.
A pulsed or continuous wave CO.sub.2 laser can be directed selectively through or around a doubler crystal for operation in different frequency bands. An electroacoustic tunable filter and detector arrangement discriminates for absorption and emission at particular frequencies characteristic of pollutant gases, and a computer decodes the timing and absorption/emission information as a function of optical wavelength and distance. The computer preferably also generates and records a profile of gas concentrations along the sight path, including the concentrations of hazardous pollutant gases.
2. Prior Art
Infrared spectroscopy is a known method for assessing concentrations of gases in samples. Systems that use a laser and an acousto-optic tunable filter are disclosed, for example, in U.S. Pat. Nos. 4,490,845--Steinbruegge et al; 4,622,845--Ryan et al; and 4,652,756--Ryan et al. The technique generally involves passing an infrared band laser beam from a source to a detector, across the flowpath of gases in a stack. Reflectors can be used to pass the beam across the stack more than once, thus increasing the extent to which the beam is affected by the sample of gases in the stack. Typically, the path of the beam is "closed," i.e., the light passes from the source, through the gas, to the detector. Gases in the stack absorb the illuminating radiation selectively at specific frequencies due to the molecular and atomic structure of the gas molecules. The detector discriminates for known patterns of absorption, i.e., absorption at certain wavelengths and not at other wavelengths.
Gas also may emit radiation at specific frequencies due to fluorescence effects following the application of sufficient excitation energy or by thermal excitation which produces blackbody radiation. Fluorescence effects are a form of reflectance. Normally, fluorescence is very low in power compared to the illuminating energy. Thus fluorescence is difficult to detect in a closed path arrangement during illumination, or in a closed path arrangement wherein the detector determines absorption as a function of wavelength in the range of illumination. Fluorescence measurements also typically are conducted at close range, to enable application of sufficient excitation energy to produce a detectable response. By analyzing the energy received as a function of frequency or wavelength, it is possible to detect the presence of particular molecules, and to assess the concentration of these molecules in the stack gases. According to the patents to Ryan et al, stack monitoring is done repetitively in an automated manner using a computer controller and analyzer for controlling a tunable filter at the receiver.
Monitoring stack gases requires a detection arrangement that is fixed and applicable only to measure the instantaneous concentration(s) of gas(es) in the stack. In conjunction with a flow measurement technique, this information can be converted into a gas volume figure that may be meaningful with respect to any air pollution at large. It would be advantageous to facilitate fast and automated measurements across open air where needed, and to provide a system with the versatility needed to discriminate for a wide variety of gases.
In general, there are five open path remote sensing techniques applicable to assessment of gas concentrations in the air. These are fluorescence, differential optical absorption spectroscopy, tunable diode laser absorption spectroscopy, differential absorption lidar spectroscopy, and Fourier transform infrared spectroscopy. These are each methods for measuring the wavelength-specific behavior of the gas molecules such that characteristic patterns that represent particular gases can be identified in the data.
The fluorescence technique measures the light intensity emitted by specific gases at characteristic wavelengths. The light is emitted when electrons in the gas molecules return to a lower energy state after the molecules have been excited, typically by radiation from a high intensity light source. Fluorescence measurement is restricted to measurements in the ultraviolet, where OH radicals and SO.sub.2 can be effectively discriminated by characteristic spectroscopic signatures. However, expensive equipment is required and the equipment is designed to measure only for specific pollutants. The technique lacks versatility and is operable only with respect to a sample that is very close to the illumination source and the detector.
Differential optical absorption spectroscopy involves measuring the differential intensities between absorption peaks and valleys versus wavelength in the ultraviolet-to-visible regions. The light source is usually a high intensity lamp and the maximum path length is around 800 m. This method has good specificity for discriminating among gases, and is the only method that effectively measures NO.sub.3 radicals. Equipment for making the measurements is readily available, for example as embodied in the OPSIS system, installed at various locations in Europe. However, because the system does not encompass the mid-to-far infrared spectral band, it is ineffective for discriminating most molecular hydrocarbon concentrations, which unfortunately include many pollutants that it would be desirable to detect.
Mid-IR tunable diode lasers are available for tunable diode laser absorption spectroscopy. A tunable light source, as opposed to a wide band light source, can simplify the equipment required for light absorption spectroscopy because the sample can be illuminated at the wavelengths of interest, and the absorption of the light at these frequencies can be examined. The tunable diode approach has high time resolution, excellent specificity, high sensitivity for NO.sub.2, and also measures HNO.sub.3, NH.sub.3, HCHO and H.sub.2 O.sub.2 at trace levels. It detects pollutants that other techniques cannot, and/or has a higher sensitivity due to precise control of illumination wavelength. However, laser diodes of sufficient power do not exist for the far-IR region where most hydrocarbon pollutants absorb. In the wavelengths where tunable diode lasers operate, power constraints of the source and sensitivity limitations of detectors limit atmospheric absorption measurements to a path length of about 300 m.
Instead of using fixed reflection targets, differential absorption lidar spectroscopy uses atmospheric backscatter of tunable pulsed lasers. This technique measures absorption and has been most successful in the ultraviolet and visible regions, where molecular scattering is prevalent. In the IR band, aerosols must provide the scattering. This technique has the advantage that range-resolved profiles over a substantial distance (e.g., 3 km) can be developed, i.e., the concentrations of detected gases as a function of distance from the source/detector. The present invention may also use a pulsed laser with a ranging capability, which enables localization and volume measurements of pollution clouds. The invention, however, is arranged to operate in the mid-to-far infrared, and uses a tunable receiver.
Fourier transform infrared spectroscopy involves interferometry. A beam from a high intensity lamp is propagated through the atmosphere and split into two beams at the receiver. One beam is directed to a fixed mirror and the other beam to a moving mirror. The two beams are recombined to form an interferogram from which the absorption spectra is obtained. This technique is useful in the two IR atmospheric transmission windows where many toxic pollutant chemicals absorb, i.e., 3.3 to 4.2 .mu.m and 8.3 to 13.3 .mu.m. The method is good for relatively high pollutant concentrations, but it is limited in that the sensitivity for most pollutants is not sufficient for ambient monitoring in moderately polluted or unpolluted areas, where it may be desirable to detect and measure for traces. Moreover, the range is limited to about 500 m.
Unless one desires to measure only for the specific type of gas and concentration range, and perhaps at a specific location for which the foregoing monitoring systems are respectively designed, more than one of them is needed to avoid the drawbacks of power, frequency and sensitivity limitations of each. It would be possible to combine all the foregoing types of monitors in one system, to provide a measurement and detection system that enjoyed the advantages of the respective techniques. This would be prohibitively expensive and complex.
According to the present invention, infrared spectroscopy techniques are applied to a directable sighting device having an automated tunable filter detector arrangement and a multi-band or wide band source having means for selectively directing an illuminating beam through a nonlinear crystal to produce harmonics. The tunable filter is preferably an acousto-optical tunable diffractor, e.g. , comprising at least one crystal of thallium arsenic selenide (Tl.sub.3 AsSe.sub.3) or any other acousto-optic material. This crystal is operable as a tunable diffractor by varying the frequency of a modulating acoustic wave passed through the crystal by application of a radio frequency modulating field.
U.S. Pat. No. 3,805,196--Feichtner et al discloses how to make and use a thallium arsenic selenide or "TAS" crystal as a controllable diffractor. The acoustic wave generated in the crystal produces alternating compression and rarefaction fronts, which have different indices of refraction. The wave fronts form a diffraction grating that spreads the spectrum of light passed therethrough, and diverts the received beam as a function of wavelength. The angle of refraction of the grating can be adjusted with the frequency of the acoustic wave, and the amount of light diffracted increases with the intensity of the acoustic wave. Therefore, by varying the acoustic frequency the crystal is tuned such that a particular wavelength can be directed on a detector. Furthermore, the angular shift of the diffracted beam can be mostly compensated by creating a wedge at either the input or output optical face of the acousto-optic tunable filter, with the result that the diffracted beam always appears at the same angle to the detector irrespective of the acoustic frequency. The output of the detector is digitized and stored to develop absorption information as a function of optical wavelength. A computer then determines the concentrations of gases along the sight path from their characteristic absorption spectra.
U.S. Pat. No. 4,505,550--Steinbruegge discloses an acousto-optic tunable filter in infrared bandwidths, useful for imaging equipment. U.S. Pat. Nos. 4,575,186--Gottlieb et al, and 4,705,362--Ryan et al disclose variations including, for example, a plurality of crystal arrangements for operating in different bands to enlarge the bandwidth of the filter as a whole. Each of the foregoing patents is hereby incorporated as if set forth in full.